The present invention relates generally to radiocommunication systems having control channels and, more particularly, to the location of digital control channels in such systems.
Radiocommunication systems have traditionally been analog in nature. The rapid growth of radiocommunication systems, however, has compelled system designers to search for ways in which system capacity can be increased without reducing communication quality beyond consumer tolerance thresholds. One way in which increased capacity can be provided is by changing from analog to digital communication techniques. In North America, this change was implemented by transitioning from the analog AMPS system to a digital system (D-AMPS) which is now standardized as IS-54B.
Since a large consumer base having equipment that operated only in the analog domain existed prior to the introduction of digital techniques, a dual-mode (analog and digital) standard was adopted in IS-54B so that analog compatibility was provided in tandem with digital communication capability. For example, the IS-54B standard provides for both analog and digital traffic channels, wherein the system operator can replace analog traffic channels with digital traffic channels, and vice-versa, in a dynamic manner to accommodate fluctuating traffic patterns among analog and digital users.
In addition to traffic channels, radiocommunication systems also provide control channels which are used to carry call setup data messages between base stations and mobile stations. According to IS-54B, for example, there are 21 dedicated analog control channels which are assigned to fixed frequencies for each of the A and B carriers. These analog control channels are termed "dedicated" since they are always found at the same frequency and, therefore, can be readily located by the mobile stations which need to monitor the data which is transmitted thereon.
For example, when in the idle state (i.e., turned on but not in use), a mobile station in an IS-54B system tunes to and then continuously monitors the strongest control channel at its known frequency (generally, the control channel of the cell in which the mobile station is located at that moment) and may receive or initiate a telephone call through the corresponding base station. When moving between cells while in the idle state, the mobile station will eventually "lose" radio connection on the control channel of the "old" cell and tune to the control channel of the "new" cell. The initial tuning to, and the change of, control channel are both accomplished automatically by scanning all the control channels at their known frequencies in operation in the cellular system to find the "best" control channel. When a control channel with good reception quality is found, the mobile station remains tuned to this channel until the quality deteriorates again. In this manner, all mobile stations are nearly always "in touch" with the system.
As such hybrid systems mature, it is anticipated that the number of analog users will diminish and the number of digital users will increase. Eventually all of the analog traffic channels will probably be replaced by digital traffic channels. When that occurs, less expensive digital-only mobile units can replace the current dual-mode units. However, such digital-only mobile units would be unable to scan the analog control channels currently provided in the IS-54B system.
Accordingly, it is desirable to provide digital control channels to radiocommunication systems which support digital technology, such as that described by IS-54B. In addition to compatibility issues, digital control channels are also desirable for other reasons described in the above-identified application, for example an enhanced sleep mode for mobile units which results in longer battery life. Whereas IS-54B provides dedicated control channels, more flexibility is desirable in assigning the number of control channels and the frequencies for these control channels to optimize system capacity and to support hierarchical cell structures, i.e., microcells, picocells, etc. If, however, the digital control channels are not located on known frequencies, the question arises as to how the remote units will be able to locate these control channels for monitoring.
One conventional radiocommunication system used in Europe, known as the GSM, is already an all-digital system. In this system, the mobile unit simply scans through all of the available channels until it identifies a digital control channel. This location technique, however, is too slow for systems having a large number of channels. Moreover, the problem of locating a digital control channel after call termination is exacerbated by handoffs of mobile units that move from cell to cell, since a mobile unit cannot then even use its knowledge of the location of the control channel which it had been monitoring prior to the call.